1. Field of the Invention
This invention relates to processes for removing metal carbonyls from gaseous streams and, in particular, from synthesis gas streams.
2. Description of the Prior Art
Synthesis gas, a gas mixture containing primarily hydrogen and carbon monoxide, has become an increasingly important feedstock for the chemical industry. The gas can be produced by the partial oxidation or steam-reforming of hydrocarbons including natural gas, distillate oils and residual oil, and by gasification of coal and coke. Synthesis gas is converted to many valuable chemicals by such catalytic processes as the Fisher-Tropsch synthesis, which produces a variety of selected alcohols, hydrocarbons, ketones and aldehydes, and the Oxo hydroformylation process in which synthesis gas is reacted with olefins to yield numerous specific alcohols, ketones and aldehydes. The conversion of synthesis gas by these catalytic processes, however, may be adversely affected by various impurities which are almost invariably present in synthesis gas.
The type and relative concentrations of impurities in synthesis gas depend on the feedstock from which the gas is derived and the conditions under which it is produced. Typically, as a result of its manufacture, synthesis gas contains impurities including sulfur compounds, such as hydrogen sulfide (H.sub.2 S), carbonyl sulfide (COS), carbon disulfide (CS.sub.2) and methyl mercaptan (CH.sub.3 SH), as well as hydrogen cyanide (HCN), hydrogen chloride (HCl) and other compounds.
Present purification schemes to remove high concentrations of such impurities generally utilize a reactive liquid absorbent, such as aqueous solutions of ethanolamines or alkali carbonates and hydroxides, as a primary purification agent. Also employed as a primary purification agent are non-reactive physical absorbents, such as methanol at cryogenic temperatures. Purifying synthesis gas to a high degree with such absorbents, however, is uneconomical due to the disproportionately large amounts of energy required to regenerate the spent absorbent.
Accordingly, after treatment by a primary purification scheme, synthesis gas generally is subjected to a further purification process to reduce the sulfur impurities to an acceptable level. For many synthesis gas uses, sulfur concentrations below 1 part per million by volume (ppmv) are required. Though absorbents such as activated charcoal and molecular sieves are useful for removing sulfur compounds, the principal absorbent employed in commercial processes to remove sulfur impurities is zinc oxide.
Employment of zinc oxide absorbents to remove sulfur impurities from gaseous streams is extensively described in the prior art. For example, U.S. Pat. Nos. 3,441,370 and 4,128,619 describe the removal of H.sub.2 S with the use of a zinc oxide absorbent. U.S. Pat. No. 4,009,009 describes the use of zinc oxide to remove COS from arsenic-free gas streams and U.S. Pat. No. 3,492,083 broadly describes the removal of H.sub.2 S and SO.sub.2 from an industrial gas by using as an absorbent a mixture comprising oxides of aluminum, zinc, iron or manganese in combination with oxides of alkaline earth or alkali metals. U.S. Pat. No. 2,239,000 discloses the removal of sulfur from gas mixtures comprised of hydrogen and carbon monoxide using catalytic mixtures of zinc and iron oxides or zinc and chromium oxides. Also, U.S. Pat. No. 4,271,133 discloses the removal of H.sub.2 S, COS, SO.sub.2 CS.sub.2, CH.sub.3 SH, HCN and HCl from gases using zinc oxide as an absorbent.
The absorption of sulfur compounds by zinc oxide has been reported to involve the conversion of zinc oxide to zinc sulfide. See "Catalyst Handbook", pages 46-56, Springer-Verlag New York, Inc. (1970). Conventionally, as zinc oxide is converted to zinc sulfide, the absorption function of zinc oxide is understood to become exhausted.
However, there has been no appreciation, heretofore, regarding the possible capacity of zinc sulfide as an effective gas stream absorbent. Therefore, the converted, or "spent" zinc sulfide-zinc oxide has simply been discarded and replaced with fresh zinc oxide absorbent. Thus, while zinc oxide is well known in the prior art as an absorbent for the removal of sulfur compounds and hydrogen cyanide, there has been no appreciation regarding its possible capacity to act as an effective absorbent either directly or as an intermediate in the removal of such nonsulfur impurities as metal carbonyls, that also may be present in synthesis gas or other gas streams.
Metal carbonyl contamination of gas streams is principally a result of exposure of the gas to various types of metallic containers, reactors and piping. It is well known that the presence of metal carbonyls, particularly iron pentacarbonyl, Fe(CO).sub.5, and nickel tetracarbonyl, Ni(CO).sub.4, and mixtures thereof, at even very minor concentrations can seriously affect the activity and selectivity of the catalytic processing of synthesis gas.
In addition, fuel gases, such as natural gas and "Town gas" (a gaseous mixture containing hydrogen, carbon monoxide and methane) from underground storage reservoirs, are often contaminated with metal carbonyls. During combustion of fuel gases, the presence of metal carbonyls can lead to the deposition of metals and metal compounds on burners thereby inhibiting efficient oxidation.
Furthermore, metal carbonyls are known to be highly toxic. For example, the maximum exposure limit to nickel carbonyl is only one part per billion. Thus, the need to remove metal carbonyls from gaseous streams is extremely important.
Many procedures have been proposed in the prior art for removing metal carbonyls from synthesis gas and other gases. For example, Canadian Patent No. 1,041,026 discloses metal carbonyl absorption on activated alumina. In the described process, however, the feed gas must be cooled, preferably to the range of 5.degree. to 15.degree. C. U.S. Pat. No. 3,433,841 discloses the use of cation exchange resins to remove iron carbonyl impurities by way of an oxidation reduction reaction. U.S. Pat. No. 1,631,823 discloses the use of activated carbon and silica gel as absorbents for iron carbonyls.
Activated carbon has also been used in processing natural gases and "Town gas" from underground reservoirs to remove by absorption the metal carbonyls contained therein. See Degent et al., "Contribution to the Study of the Formation, Elimination and Analysis of Traces of Iron Carbonyl and Nickel Carbonyl in the Gas of the Beynes Underground Reservoir," Compute Rendu Assoc. Tech. de l'Ind. du Gaz en France (1961). However, this reference teaches that activated carbon is inefficient in removing nickel carbonyls.
Mixing the metal carbonyl-containing gas with a small amount of air, or other oxidizing gas increases the efficiency of nickel carbonyl removal by forming metal oxides. See Dumay et al., "How the Problem of Eliminating Nickel Carbonyl From the Gas Taken Back From the Underground Storage At Beynes Was Solved," Assoc. Tech. de l'Ind. du Gaz en France (1965). However, when metal oxides are absorbed on activated carbon, they must be removed to regenerate the activated carbon. Their removal has proven so difficult that the exhausted activated carbon is generally discarded and replaced. In industrial operations, the relatively high cost of activated carbon and problems associated with spent carbon disposal make such metal carbonyl removal methods unattractive.
In U.S.S.R. Patent No. 473,509, chemisorbent sulfhydryl filaments are proposed for absorbing nickel carbonyl. The hydrogen or mercury ion forms of sulfhydryl are required in the described process. Indeed, it has been long known that the use of sulfur compounds by themselves was not suitably efficient in removing metal carbonyls. See Degent et al., "Contribution to the Study of the Formation, Elimination and Analysis of Traces of Iron Carbonyl and Nickel Carbonyl in the Gas of the Beynes Underground Reservoir," Compute Rendu Assoc. Tech. de l'Ind. du Gaz en France (1961). Though removal of nickel carbonyl by passing "Town gas" through a sulfur-containing liquid was said to be achieved, such removal has not been found reproducible. Cooper et. al., "Nickel and Iron Carbonyls In Town Gas" The Gas Council, London (1963).
It has now been found that zinc sulfide is an effective absorbent for reducing the metal carbonyl content of gaseous streams.
Therefore it is an object of the present invention to provide a novel process of employing zinc sulfide as an absorbent for removing metal carbonyl impurities from gaseous streams.
Another object of this invention is to provide a process for removing metal carbonyl impurities from gases using zinc sulfide-zinc oxide as the absorbent.
Still another object of this invention is to provide a process for removing metal carbonyl impurities from gaseous streams that is efficient over a wide range of operating conditions.
Other objects and advantages of the present invention will become apparent from the following description of the invention.